1. Field of the Invention
The present invention relates generally to the field of ink compositions with special applicability to the field of lithographic compositions or oil based compositions usable in lithography and to the field of nonlithographic compositions in which the relationship of such compositions with aqueous solutions, including its solubility, can be selectively controlled. The compositions of the present invention are unaffected by water, and thus generally water insoluble, at certain selected acidic pH levels and water washable at certain other selected alkaline pH levels. The present invention also relates to methods of making, using and recovering such compositions.
2. Summary of the Prior Art
A wide variety of printing processes currently exist in the art. Although the ink composition and method of the present invention will have applicability to most if not all of these prior processes, it has particular applicability to a lithographic printing process commonly referred to as lithography. Lithography is a method of printing which relies on differences in solubility and surface wetability between an oil based component and an aqueous or aqueous based component to effectively transfer the printing ink to the desired image area and prevent it from transferring to the nonimage areas.
The printing apparatus commonly used in a lithographic process includes a printing plate which is treated to provide an oleophilic ink-accepting image area and an oleophobic ink-repelling nonimage area. Generally, the oleophilic or oil attracting image areas are hydrophobic or water repelling, and the oleophobic or oil repelling nonimage areas are hydrophilic or water attracting.
During a conventional lithographic printing process, an oil based or water insoluble ink composition and an aqueous fountain solution are applied to the printing plate. Because of the solubility and the wetability differences of the oil based and aqueous compositions, the fountain solution is preferentially attracted to and preferentially wets the oleophobic nonimage areas, while the ink is preferentially attracted to and preferentially wets the oleophilic image areas. It is well known and accepted in the art that successful lithographic printing requires inks which exhibit stability relative to water and thus remain water insoluble. Absence of this characteristic will result in poor print quality, poor edge definition, dot gain and various other print deficiencies. Thus, lithographic inks are selected and formulated for their ability to remain stable, cohesive and insoluble when contacted with water. The simultaneous feeding of the ink composition and fountain solution to the print plate is accomplished through a variety of methods and roller configurations known in the art.
After the ink composition and fountain solution have been applied to the printing plate and allowed to gravitate to their respective oleophilic and oleophobic areas, they are transferred either directly to an image receiving paper or other print substrate or to an intermediate blanket cylinder which then subsequently transfers the ink to the paper. The former is known in the industry as direct plate to paper lithographic printing, while the latter is generally referred to as offset lithography. In offset lithography, the blanket cylinder is covered with a transfer substrate typically made of rubber which receives the ink from the printing plate and transfers it to the print substrate.
In the typical lithographic printing system described above, the ink pigment is carried by the oil based component while the fountain solution comprises the aqueous component. Although there has been little commercial application to date, the opposite could also be used. In other words, the image areas could be oleophobic (or hydrophilic) with the ink pigment being carried by the aqueous component and the nonimage areas could be oleophilic (or hydrophobic) with the fountain solution being oil based. In either system, the lithographic process, by definition, must include both an oil based or water insoluble component and an aqueous component. Certain aspects of the present invention are applicable to either system. Thus, the present invention relates broadly to an improved oil based or water insoluble component or composition for use in a lithographic or other printing process.
During the course of lithographic printing, the printing plates are periodically changed as one job is completed and another started. Whenever this occurs, the blanket cylinder in an offset process must be cleaned to remove ink residue which is present from the previous job. Further, if a change of ink is desired, the entire print train including the application rollers, the print plate and the blanket must be cleaned. Such cleaning is commonly accomplished using an appropriately formulated wash solvent. To be effective as a wash solvent, the wash must be compatible with, or be able to dissolve or sufficiently disperse the ink. Since the inks are oil based or water insoluble, this necessitates the use of organic or petroleum based or other non-aqueous wash solvents to effectively remove the ink from the rollers, printing plates, blanket cylinder, etc.
These organic wash solvents can give rise to employee safety concerns and are a large source of both air and water pollution as volatile organic compounds (VOC's) are dispersed into the atmosphere or disposal systems. Such pollution is due to evaporation into the ambient air or into a venting system during the washing of the print components as well as the disposal or laundering of shop towels and rags used in the cleaning process. Attempts to develop water based wash or cleaning solutions have not been successful due to the inherent ability of the ink to resist water. Attempts have also been made to use water/solvent mixtures by emulsifying petroleum-based solvents into water through the use of emulsifiers and surfactants, but these products suffer from inherent instability due to immiscibility of water and the solvents and perform slowly. Further, such mixtures do not completely eliminate the use of petroleum solvents which are still commonly present in an amount of about 30-80%. Other industry trends include the use of solvents such as terpenes. While not petroleum based, their performance has been marginal and they are costly and in short supply. In addition, their use and disposal also pose environmental concerns.
Thus, although offset lithography is recognized and established as a dominant printing process for certain applications, drawbacks exist because of the pollution problems referred to above. These are becoming more of a drawback and more of a problem as new pollution control regulations and standards are mandated. In fact, because of the very nature of the lithographic process at least one of the components (either the ink composition or the fountain solution) must be oil based or water insoluble. This in turn necessitates the use of an organic or petroleum based solvent to wash the apparatus. This is generally accepted as a necessary limitation of the lithographic process about which little can be done.
For nonlithographic applications, various water based or water soluble inks are currently available. Some of these utilize water dispersible or soluble resins which have been preneutralized to provide such properties. See for example U.S. Pat. No. 4,966,628 issued on Oct. 20, 1990 to Amos et al. While these inks can be cleaned up with water or water solutions and thus reduce pollution concerns, it is generally recognized that existing water soluble or dispersible inks exhibit inferior water and alkaline resistance as well as inferior drying, adhesion and gloss properties when compared to conventional oil based inks.
Radiation curable inks have been used, both lithographically and nonlithographically, to reduce the release of VOC's common to many oil based inks. However, despite the recognized advantages of radiation curable inks over ink compositions which set and/or dry by solvent evaporation, certain limitations continue to exist. For example, although radiation curable inks can be formulated with little if any solvent component, the components of conventional radiation curable ink compositions are not water soluble to the extent that they can be washed or cleaned up with aqueous solutions. Thus, the cleanup of radiation curable inks from printing plates, rollers, brushes, blankets and other application equipment necessarily involves the use of nonaqueous, organic or other solvents which are capable of dissolving or washing, and thus cleaning up, the various resins and other components of the ink composition. In many cases, these solvents are volatile organic compounds such as glycol ethers or esters. Some of these are considered as hazardous wastes, thereby creating obvious safety concerns and disposal problems. Others, even though not technically considered as hazardous wastes, still pose serious health and safety concerns. Various attempts have been made to overcome this problem by formulating water soluble radiation curable inks which are substantially water soluble and can thus be cleaned up with aqueous solutions. However, such water based or water soluble inks generally exhibit properties inferior to certain desired properties of water insoluble inks and do not print lithographically.
Accordingly, there is a need in the art to address the above problems and to provide an ink composition, and related methods of making, using and recovering, which substantially reduces if not eliminates the above pollution concerns. There is also a need in the art for ink compositions which, in addition to addressing pollution concerns, results in highly acceptable print quality and provides improved ink properties relating to ink set, ink life and the reduction of volatile solvents.